Dehydrogenation reactions have been used to effect aromatization as well as to place an olefinic linkage in an open chain system (see, P. A. Plattner, in "Newer Methods of Preparative Organic Chemistry", Wiley Intersciences, New York, pp. 21-59 (1948); Chem. Rev. 78: 317-361 (1978)). Two types of reagents are most frequently used to effect aromatization: (a) hydrogenation catalysts such as platinum, palladium, nickel, etc., and (b) sulfur or selenium which combine with the hydrogen evolved to give H.sub.2 S or H.sub.2 Se.
Other reagents have also been used for dehydrogenation, e.g., atmospheric oxygen, selenium dioxide, quinones (see, Jackman, L. M., Org. Chem. 2: 329-366 (1960)) and activated charcoal (see, Shuikin and Naryschkina, J. Prakt. Chem. [4] 13: 183 (1961).
Dehydrogenation of open-chain systems to give a double bond in a specific location is not usually a feasible process, though industrially mixtures of olefins are obtained in this way from alkanes.
There are few highly selective methods for catalytic dehydrogenation. Competing secondary reactions such as hydrogenolysis of carbon-oxygen bonds may occur detracting from the synthetic utility of this approach. For example, during the catalytic dehydrogenation of cyclohexylhydroquinone to phenylhydroquinone using a supported palladium catalyst, large amounts of hydrogenolysis by-products such as 2- and 3-phenyl phenols are produced.
Thus, it is apparent there is still a need for a catalyst that will effect a selective dehydrogenation of substrates that are susceptible to hydrogenolysis.